Method for contention based random access on a secondary carrier

ABSTRACT

A mobile station performs random access method. The method includes receiving, from a first cell, a message identifying a random access preamble to transmit to a second cell, wherein the first and second cell operate on different frequencies. The method also includes determining whether the random access preamble is from a set of contention based random access preambles. In addition, the method includes receiving a random access response message, wherein the random access response message is addressed to a unique identifier of the mobile station; and transmitting, if the identified random access preamble is from the set of contention based preambles, a timing verification message.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims benefits under 35 U.S.C. 119 to copendingU.S. provisional Application No. 61/543,135 filed on 4 Oct. 2011, thecontents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to communication systems and inparticular, to random access when multiple timing advances are used.

BACKGROUND

Carrier Aggregation will be used in future LTE networks to provideimproved data rates to users. Carrier aggregation consists oftransmitting data to or receiving data from the UE on multiple carrierfrequencies (“component carriers”). The wider bandwidth enables higherdata rates.

A UE can be configured with a set of component carrier (CCs).Specifically, the UE is configured with a cell on each componentcarrier. Some of these cells may be activated. The activated cells canbe used to send and receive data (i.e., the activated cells can be usedfor scheduling). The UE has up to date system information for allconfigured cells. Therefore, after a cell has been configured, it can bequickly activated. Thus, when there is a need for aggregating multipleCCs (e.g., a large burst of data), the network can activate configuredcells on one or more of the CCs. There is a designated primary servingcell (Pcell) on a CC that is referred to as the primary CC, which isalways activated. The other configured cells are referred to assecondary serviong cells (Scells), and the corresponding CCs arereferred to as secondary CCs.

Remote Radio Heads (RRHs) are used to extend coverage of a base station.As part of the work on carrier aggregation, next-generation cellularcommunication systems will support carrier aggregation of frequencies onwhich RRHs are deployed. Carrier Aggregation will be used to provideimproved data rates to users. Carrier aggregation consists oftransmitting data to or receiving data from the user equipment (UE) onmultiple carrier frequencies (“component carriers”). The wider bandwidthenables higher data rates.

RRHs are deployed on a different frequency than the frequency used bythe base station site and provide hot-spot like coverage on thatfrequency. User equipment (UE) that is in such a hot-spot can performcarrier aggregation of the frequency used by the base station and thefrequency used by the RRH and obtain corresponding throughput benefits.RRHs do not embody typical base station functionalities such as higherlayer processing, scheduling etc. The baseband signal transmitted by anRRH is generated by the base station and is carried to the RRH by a highspeed wired (e.g., optical) link. Thus RRHs function as remote antennaunits of a base station, with a high speed link to the base station.

A base station 101, RRH 102, and UE 103 are shown in FIG. 1. As isevident, a non-wireless link 104 exists between base station 101 and RRH102. The transmissions to UE 102 occur both from base station 101 andfrom RRH 102, except that the transmissions from base station 101 existon a different frequency than the transmissions from RRH 102.

The presence of RRHs introduces additional physical locations from whichthe UE can receive the base station signal (i.e., in addition toreceiving the base station signal directly from the base station). Inaddition, there is a delay introduced by the communication between thebase station and the RRH. This delay results in the UE perceiving verydifferent propagation delays on the frequency used by the base stationand the frequency used by the RRH. As a consequence, the timing advanceapplied to the two frequencies need to be different.

FIG. 2 shows the timing relationships between downlink and uplinktransmissions of the two frequencies. In particular, downlink (DL)transmission (Tx) is shown on frequency 1 (F1) as subframe 201, DLreception (Rx) is shown on F1 as subframe 202, UL Tx is shown on F1 assubframe 203, UL Rx is shown on F1 as subframe 204. In a similar mannerDL Tx is shown on F2 as subframe 205, DL Rx is shown on F2 as subframe206, UL Tx is shown on F2 as subframe 207, and UL Rx is shown on F2 assubframe 208.

It is assumed that base station 101 tries to ensure that uplinktransmissions on F1 and F2 are received at the same time. Transmissionson F2 through RRH 102 (both uplink and downlink) have an additionaldelay due to transmission through fiber link 104 and the associated RRHprocessing. This additional delay can be as large as 30 microseconds. Asshown in FIG. 2, in order for the F2 uplink to arrive at the basestation at the same time as the F1 uplink, the timing advance applied bythe UE for transmissions on F2 has to compensate for the fiber and RRHprocessing delay.

As a result, the uplink subframes 203, 204, 207, and 208 on F1 and F2are not time aligned. In FIG. 2, F2 uplink subframe 207 starts before F1uplink subframe 203.

There may also be a need for a different frame timing when the twocarriers are on different bands with a large frequency separation, evenwithout deployment of RRHs. In such cases the UE has to maintain aseparate timing advance for the second carrier.

In order to obtain a timing advance for the primary carrier, the UE hasto perform a random access procedure on the primary carrier. The UEtransmits a random access preamble to the eNB on the primary uplinkcarrier frequency. The eNB calculates the appropriate timing advance theUE should apply based on the timing of the received random accesspreamble. The eNB transmits a Random access response message (RAR) inresponse to a RACH preamble transmission by the UE, on the primarydownlink carrier frequency. The RAR includes the timing advancecalculated by the eNB from the RACH transmission.

In order to maintain uplink timing on a secondary carrier, the UE has toperform a random access preamble transmission on the secondary carrier.Transmitting the RAR message on the downlink of the secondary carrier isa possibility. However, such a mechanism is inadequate, as discussedbelow.

Support of Heterogeneous Network Scenarios:

If an LTE UE is experiencing control channel interference on the SCell(for example, due to presence of interfering pico cells on the secondaryCC), then the UE is unable to receive PDCCH transmission from the eNB onthe SCell. Receiving the RAR message requires the UE to be able toreceive the PDCCH for the RAR message, which the UE is unable to do inthis case. In such a case, the eNB configures “cross carrier scheduling”for the SCell: the PDCCH for the SCell is transmitted on a differentcell (e.g., PCell). This enables the eNB to use the SCell for PDSCHtransmissions. However, since there isn't a mechanism to resynchronizeuplink timing, the UE will be unable to use the SCell uplink.

Additional PDCCH Blind Decodes:

In LTE, the downlink control information is transmitted via the PhysicalDownlink Control channels (PDCCH). The PDCCH typically contains controlinformation about the downlink control information (DCI) formats orscheduling messages, which inform the UE of the modulation and codingscheme, transport block size and location, precoding information,hybrid-ARQ information, UE Identifier, Carrier Indicator Function, CSIrequest fields, SRS request field, etc. that is required to decode thedownlink data transmissions or for transmitting on the uplink. Thiscontrol information is protected by channel coding (typically, acyclic-redundancy check (CRC) code for error detection and convolutionalencoding for error correction) and the resulting encoded bits are mappedon the time-frequency resources. For example, in LTE Rel-8, thesetime-frequency resources occupy the first several OFDM symbols in asub-frame. A group of four Resource Elements is termed as a ResourceElement Group (REG). Nine REGs comprise a Control Channel Element (CCE).The encoded bits are typically mapped onto either 1 CCE, 2 CCEs, 4 CCEsor 8 CCEs. These four are typically referred to as aggregation levels 1,2, 4 and 8. The UE searches the different hypotheses (i.e., hypotheseson the aggregation level, DCI Format size, etc) by attempting to decodethe transmission based on allowable configurations. This processing isreferred to as blind decoding.

To limit the number of configurations required for blind decoding, thenumber of hypotheses is limited. For example, the UE does blind decodingusing the starting CCE locations as those allowed for the particular UE.This is done by the so-called UE-specific search space (UESS), which isa search space defined for the particular UE (typically configuredduring initial setup of a radio link and also modified using RRCmessage). Similarly a common search space (CSS) is also defined that isvalid for all UEs and might be used to schedule broadcast downlinkinformation like Paging, or Random access response, or other purposes.The number of blind decoding attempts that a UE performs is limited(e.g. 44 in Rel-8 LTE (12 in CSS and 32 in UESS), and up to 60 for Pcell(12 in CSS, and 48 in UESS) and up to 48 for Scell (48 in UESS) inRel-10) for several reasons.

Other than reducing the computational load (i.e. convolutional decodingattempts), limiting the number of blind decodes also help in reducingCRC falsing rate. A CRC falsing occurs when the UE decodes an incorrecttransmission and treats it as a valid PDCCH because the CRC passes andthis can lead to protocol errors or other errors resulting in systemperformance loss. Therefore, it is desirable to keep the falsing ratesvery low. Typically, for a k-bit CRC attached, if n is the number ofdecoding attempts, the probability of CRC falsing (i.e. false positive)is approximately n×2^(−k).

For carrier aggregation operation in LTE, the system information istypically transmitted via the Pcell and hence the UE monitors the CSSand UESS for the Pcell. The Scell system information is transmitted viaRRC signaling (on a UE specific basis) and this can be transmitted viathe Pcell and there is no need for the UE to monitor the CSS on theScells. Thus, the CSS corresponding to the SCell are not monitored andthe UE benefits from having to perform smaller number of blind decodesfor the Scells. The RAR is transmitted using an RA-RNTI (i.e., the PDCCHfor the RAR is scrambled using an RA-RNTI). RA-RNTI is a broadcastidentifier and reception of the PDCCH for RAR requires monitoring of thecommon search space on the SCell DL. As described above, monitoring thecommon search space on the SCell DL requires additional blind decodes,which increases the complexity of the UE. Therefore a procedure toacquire SCell UL timing without requiring monitoring the PDCCH commonsearch space on the SCell is needed.

There are two types of random access procedures—contention based randomaccess (CBRA) and contention free random access (CFRA). In thecontention based random access procedure, the UE selects a random accesspreamble and contention resolution is performed. The CBRA procedure isillustrated in FIG. 4. UE (401) transmits a random access preamble (411)to the eNB (402). The eNB computes the timing advance of the UE andtransmits a Random access response (RAR) message (412). The RAR messageincludes the timing advance computed by the eNB and an uplink resourceallocation (uplink grant). The UE transmits a contention resolutionmessage (413) using the uplink resources granted in the RAR message. TheeNB responds with a contention resolution message (414) which indicateswhether contention resolution is successful. It is possible that two UEsselect the same random access preamble and transmit the random accesspreamble at the same time. In this case, both UEs attempt to transmittheir respective contention resolution messages (413) using the uplinkgrant. The contention resolution message (413) includes a uniqueidentifier of the UE. If the eNB is able to decode both the contentionresolution messages, it can select one of the two UEs as the winner ofthe contention resolution and transmit the contention resolution message(414) indicating the winner of the contention resolution. The other UEcan reattempt the random access procedure.

The CFRA procedure is illustrated in FIG. 5. The eNB (502) transmits anRA preamble assignment message (510) to the UE (501). The RA preambleassignment message assigns a random access preamble to the UE. The UEtransmits the assigned random access preamble (511) to the eNB. The eNBcomputes the timing advance of the UE and transmits a Random accessresponse (RAR) message (512). The RAR message includes the timingadvance computed by the eNB. Given that the eNB assigns the preamble tothe UE, the eNB can ensure that there is a single UE using the preambleat a given time. Consequently, there is no need for contentionresolution in the CFRA procedure.

Contention free random access (CFRA) requires the eNB to reserve morepreambles to support the additional RACH transmissions for Scell RACHwhen UEs capable of uplink aggregation are present (in addition to RACHtransmissions for handover, DL data arrival). In general, CFRA ishelpful when it is important for the RACH procedure to complete quickly.For SCell RACH, quick completion is not as critical. Not having toreserve RACH preambles is a more critical requirement.

Thus there is a need to support contention based random access to obtainuplink timing on SCells, while also overcoming the deficiencies relatedto receiving the RAR message on the same cell on which the RACH preambleis transmitted.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a communication system employing a remote radio head.

FIG. 2 illustrates timing of uplink and downlink transmissions.

FIG. 3 is a block diagram showing a mobile unit.

FIG. 4 illustrates a contention based random access procedure.

FIG. 5 illustrates a contention free random access procedure.

FIG. 6 illustrates an embodiment of the invention.

FIG. 7 illustrates a procedure at a mobile station according to anembodiment of the invention.

FIG. 8 illustrates a procedure at a base station according to anembodiment of the invention.

FIG. 9 illustrates an operation of the invention.

FIG. 10 illustrates an operation of the invention.

FIG. 11 illustrates an embodiment of the invention

FIG. 12 illustrates operation within a mobile station according to anembodiment of the invention.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions and/or relative positioningof some of the elements in the figures may be exaggerated relative toother elements to help to improve understanding of various embodimentsof the present invention. Also, common but well-understood elements thatare useful or necessary in a commercially feasible embodiment are oftennot depicted in order to facilitate a less obstructed view of thesevarious embodiments of the present invention. It will further beappreciated that certain actions and/or steps may be described ordepicted in a particular order of occurrence while those skilled in theart will understand that such specificity with respect to sequence isnot actually required. Those skilled in the art will further recognizethat references to specific implementation embodiments such as“circuitry” may equally be accomplished via either on general purposecomputing apparatus (e.g., CPU) or specialized processing apparatus(e.g., DSP) executing software instructions stored in non-transitorycomputer-readable memory. It will also be understood that the terms andexpressions used herein have the ordinary technical meaning as isaccorded to such terms and expressions by persons skilled in thetechnical field as set forth above except where different specificmeanings have otherwise been set forth herein.

DETAILED DESCRIPTION

In order to meet the above-mentioned need, a method to perform acontention based random access procedure on an SCell is provided. Themethod enables the UE to obtain a timing advance for an SCell withouthaving to receive the control channel for the RAR message on the SCell.Furthermore, it eliminates the need to reserve contention freepreambles, by the eNB, for SCell random access.

Turning now to the drawings, where like numerals designate likecomponents, FIG. 3 is a block diagram showing user equipment 300. Asshown, user equipment 300 comprises logic circuitry 301, receivecircuitry 302, and transmit circuitry 303. Logic circuitry 101 comprisesa digital signal processor (DSP), general purpose microprocessor, aprogrammable logic device, or application specific integrated circuit(ASIC) and is utilized to accesses and control transmitter 303 andreceiver 302. Receive and transmit circuitry 302-303 are commoncircuitry known in the art for communication utilizing a well knowncommunication protocol, and serve as means for transmitting andreceiving messages.

User equipment 300 may aggregate carriers as described above. Moreparticularly, UE 300 supports carrier aggregation of frequencies onwhich RRHs are deployed. Thus, UE 300 will have the capabilities tocommunicate simultaneously over various frequencies to base station 101.

An embodiment of the invention is illustrated in FIG. 6. A UE (601) isconfigured to operate on a PCell (610) on a primary frequency of an eNB(602) and on an SCell on a secondary frequency (611) of the eNB (602).The eNB determines a need for the UE to perform a random accessprocedure on the SCell (611). The eNB transmits an RA preambleassignment message for the SCell (620) to the UE. The RA preambleassignment message indicates a random access preamble that the UE isrequired to transmit to the SCell. The UE transmits the random accesspreamble indicated in 620 to the SCell (621). The eNB detects the randomaccess preamble transmission and computes the timing advance for the UEfor operation on the SCell. The eNB then transmits a Dedicated RARmessage (622) to the UE. The Dedicated RAR message includes the timingadvance calculated by the eNB and can include an UL grant for atransmission by the UE to the SCell (611). The Dedicated RAR message istransmitted using a unique identifier of the UE. For example, thededicated RAR message can have a control channel that is scrambled usingthe C-RNTI of the UE. Thus, the dedicated RAR message (622) is onlyreceived by the UE (601) for which it is intended. The UE (601) appliesthe timing advance indicated in the RAR message.

The UE determines whether the random access preamble assigned at 620 isa CBRA preamble or a CFRA preamble. If the preamble is a CBRA preamble,the UE transmits a timing verification message (624) to the SCell (611)using the UL grant included in the dedicated RAR message. If the timingadvance applied by the UE is correct and if the timing verificationmessage is received by the eNB, the eNB transmits a timing confirmationmessage (625). If the preamble is a CFRA preamble, the steps 624 and 625may not be needed.

According to another embodiment, the SCell (611) may be configured forcross carrier scheduling for the UE (601). That is, control channels fordata transmissions on the SCell (611) for the UE are transmitted on acell other than the SCell (611). For example, the physical downlinkcontrol channel (PDCCH) for the SCell may be transmitted and received onthe PCell. Specifically, the dedicated RAR message (622) transmissioncan be such that the PDCCH for the dedicated RAR message is transmittedand received on the PCell and the physical downlink shared channel(PDSCH) portion of the dedicated RAR message is transmitted and receivedon the SCell. Consequently, the UE is able to receive an RAR message ona different configured cell than the one on which the random accesspreamble transmission was performed.

FIG. 7 illustrates a procedure at the UE, according to a furtherembodiment. At 701 the UE receives an order to transmit a specific RApreamble on the SCell. Such an order can be a PDCCH order including anindication of the preamble to transmit and the cell on which to transmitthe preamble. The UE transmits the RA preamble on the SCell at 702. At703 the UE determines whether the RA preamble is a CFRA preamble or aCBRA preamble. The UE can determine whether the RA preamble is a CFRApreamble or a CBRA preamble by acquiring the system information of theSCell. The system information of the SCell can indicate which preamblesare CBRA preambles and which preambles are CFRA preambles.Alternatively, the eNB can indicate whether the RA preamble is a CBRApreamble or a CFRA preamble. For example, the eNB can include in thePDCCH order at 701 an indication that the preamble is CBRA or CFRApreamble.

If the RA preamble is a CFRA preamble, the UE receives a dedicated RARmessage at 711. The dedicated RAR message includes timing advanceinformation. The UE applies the timing advance. The dedicated RARmessage is addressed to a unique identifier of the UE, such as the UE'sC-RNTI. At 712, the procedure is considered successful.

If the RA preamble is a CBRA preamble, the UE receives a dedicated RARmessage at 721. The dedicated RAR message includes timing advanceinformation and an uplink grant for the SCell. The dedicated RAR messageis addressed to a unique identifier of the UE, such as the UE's C-RNTI.The UE applies the timing advance. At 722, the UE transmits a timingverification message. The timing verification message is transmittedusing the uplink resources of the SCell indicated in the dedicated RARmessage. The timing verification message may include an identifier ofthe UE. Alternatively, the timing verification message may be a messagecarrying user data or data from upper layers. The timing verificationmessage may also be a message with predefined content.

At 723 the UE determines whether a timing confirmation message isreceived. The timing confirmation may be a message addressed to a uniqueidentifier of the UE. Alternatively, the timing confirmation message maybe an acknowledgement to the timing verification message transmitted at722. For example, the timing confirmation message may be a physicallayer acknowledgement to the timing verification message. If a timingconfirmation message is received, the procedure ends successfully at731. If a timing confirmation message is not received, the procedure isunsuccessful at 741. If the procedure is unsuccessful at 741, the UEstops using the timing advance received at 721.

Alternatively, the UE may receive a timing verification failure message,in which case the procedure is unsuccessful. Consequently, the UE stopsusing the timing advance received at 721.

FIG. 8 illustrates an eNB procedure, according to another embodiment. At801, the eNB determines a need for the UE to perform random access onthe SCell. At 802, the eNB determines whether a CFRA preamble for theSCell is available. If a CFRA preamble for the SCell is available, theeNB selects the CFRA preamble. At 811 the eNB transmits an order to theUE to transmit the selected CFRA preamble to the SCell. At 812, the eNBdetermines whether a transmission of the selected preamble is detected.If a transmission of the selected preamble is detected, the eNBcalculates a timing advance based on the received preamble transmission,and at 831 transmits a dedicated RAR message to the UE. The dedicatedRAR message includes the timing advance calculated by the eNB.

If a transmission of the selected preamble is not detected, theprocedure fails at 821. The eNB can reattempt the procedure.

At 802, if a CFRA preamble for the SCell is not available, the eNBselects a CBRA preamble for the SCell. At 841 the eNB transmits an orderto the UE to transmit the selected CBRA preamble to the SCell. At 842the eNB determines whether a transmission of the selected preamble isdetected. If a transmission of the selected preamble is detected, theeNB calculates a timing advance based on the received preambletransmission, and at 861 transmits a dedicated RAR message to the UE.The dedicated RAR message includes the timing advance calculated by theeNB and an uplink grant for the SCell.

At 862 the eNB determines whether a timing verification message isreceived on the SCell. The eNB further determines whether the timingverification message uses a correct timing advance. The eNB maydetermine that the timing advance used for the timing verificationmessage is correct based on determining that the transmission of thetiming verification message does not cause significant interference toother uplink transmissions on the SCell. The eNB may further verify thatthe timing verification message is transmitted using uplink resourcessignaled in the uplink grant at 861. If a timing verification messagethat uses a correct timing advance and uplink resources is received, theeNB at 881 transmits a timing confirmation message to the UE. The timingconfirmation may be a message addressed to a unique identifier of theUE. Alternatively, the timing confirmation message may be anacknowledgement to the timing verification message transmitted at 822.For example, the timing confirmation message may be a physical layeracknowledgement to the timing verification message.

If, at 862, the eNB determines that a timing verification message thatuses a correct timing advance and uplink resources is not received, itmay transmit a timing advance failure indication to the UE.Alternatively, the eNB may not transmit any message to the UE toindicate that a timing verification message that uses a correct timingadvance and uplink resources is not received. Consequently, theprocedure fails at 872. The eNB can reattempt the procedure.

An operation of the invention is illustrated in FIG. 9 and FIG. 10. UE1is configured with a PCell operating on a frequency CC1 and a SCelloperating on a frequency CC2. The timing advance required the uplinkoperation on PCell and the SCell are substantially different. UE1 isfurther configured for cross carrier scheduling on the SCell. Thecontrol channel for data transmissions on the SCell are transmitted onthe PCell. That is, UE1 monitors the downlink frequency of the PCell for(a) PDCCHs indicating PDSCH resource allocation on the downlink of thePCell, (b) PDCCHs indicating PUSCH resource allocation on the uplink ofthe PCell, and (c) PDCCHs indicating PDSCH resource allocation on thedownlink of the SCell.

In order to transmit PUSCH on the uplink of the SCell, the UE needs toobtain a timing advance for the SCell. At 900 UE1 receives a PDCCH orderordering the UE to perform a random access preamble transmission on theSCell. The PDCCH order indicates the random access preamble X totransmit. Preamble X is a CBRA preamble of the SCell. At 901 UE1transmits the indicated random access preamble on the SCell uplink.Another UE, UE2, may be operating on CC2. UE2 can choose the randomaccess preamble X and perform the random access preamble transmission(911) at the same time as UE1's random access preamble transmission(901). Thus the eNB may receive two overlapping transmissions of thepreamble X. Timing advance compensates for the propagation delay betweena UE and the eNB and is calculated by the eNB based on the timing of thereceived preamble transmission. Therefore, it is important to use UE1'spreamble transmission to determine the timing advance for UE1 and UE2'spreamble transmission to determine timing advance for UE2. In a scenariowhere the eNB receives two UEs transmit the same preamblesimultaneously, the timing advance calculated by the eNB may be correctonly for one of the two UEs.

According to the scenario in FIG. 9, the transmission of preamble X byUE1 (901) is received by the eNB, but the transmission of preamble X(911) is not received by the eNB.

At 902, UE1 receives a dedicated RAR message. The dedicated RAR messagehas a PDCCH that is addressed to a unique identifier of UE1, such asUE1's C-RNTI. In normal operation, any UE other than UE1 is not expectedto be able to receive and decode the PDCCH addressed to a uniqueidentifier of UE1. The dedicated RAR message includes timing advanceinformation and an uplink resource grant for the SCell. UE1 applies thetiming advance signaled in the dedicated RAR message. At 903, UE1transmits a timing verification message on the SCell according to theuplink resource grant in the dedicated RAR message. At 904, the UEreceives a timing confirmation message, which successfully completes therandom access procedure for UE1.

At the time of reception of the preamble (901, 911), the eNB cannotdetermine whether the preamble transmission is from UE1 or from adifferent UE. Therefore, optionally, the eNB may transmit an RAR messageon the downlink of the SCell (912). The PDCCH of the RAR message 912 isaddressed to a broadcast RNTI (such as a random access RNTI). The RARmessage 912 can include timing advance information and an uplink grant.The timing advance value is computed based on the received preamble X atthe eNB and is the same value included in the dedicated RAR message 902.The RAR message 912 may also indicate that it is a response totransmission of preamble X. UE2 may receive the RAR message 912 andapply the timing advance. UE2 may then transmit a contention resolutionmessage 913. The contention resolution message can include a uniqueidentifier of UE2. Given that the timing advance in RAR message iscalculated based on UE1's transmission of preamble X, the eNB may not beable to receive and decode the contention resolution message.Consequently, UE2 may not receive a message indicating successfulcontention resolution and thus fail contention resolution.Alternatively, the eNB may receive and decode the contention resolutionmessage and determine that the timing advance used by UE2 is incorrect.Consequently, UE2 may either not receive a message indicating successfulcontention resolution and thus fail contention resolution, or it mayreceive a message indicating contention resolution failure (914) andthus fail contention resolution.

FIG. 10 illustrates the scenario in which UE2's transmission of preambleX is received by the eNB and UE1's transmission of preamble X is notreceived by the eNB. UE1 is configured with a PCell operating on afrequency CC1 and a SCell operating on a frequency CC2. The timingadvance required the uplink operation on PCell and the SCell aresubstantially different. UE1 is further configured for cross carrierscheduling on the SCell. The control channel for data transmissions onthe SCell are transmitted on the PCell. At 1000 UE1 receives a PDCCHorder ordering the UE to perform a random access preamble transmissionon the SCell. The PDCCH order indicates the random access preamble X totransmit. Preamble X is a CBRA preamble of the SCell. At 1001 UE1transmits the indicated random access preamble on the SCell uplink.Another UE, UE2, may be operating on CC2. UE2 can choose the randomaccess preamble X and perform the random access preamble transmission(1011) at the same time as UE1's random access preamble transmission(1001). The transmission of preamble X by UE1 (1001) is not received bythe eNB, but the transmission of preamble X (1011) is received by theeNB.

At 1002, UE1 receives a dedicated RAR message. The dedicated RAR messagehas a PDCCH that is addressed to a unique identifier of UE1, such asUE1's C-RNTI. In normal operation, any UE other than UE1 is not expectedto be able to receive and decode the PDCCH addressed to a uniqueidentifier of UE1. The dedicated RAR message includes timing advanceinformation and an uplink resource grant for the SCell. UE1 applies thetiming advance signaled in the dedicated RAR message. At 1003, UE1transmits a timing verification message on the SCell according to theuplink resource grant in the dedicated RAR message. Given that thetiming advance value is computed based on the preamble X transmission byUE2, the eNB may not be able to receive and decode the contentionresolution message. Consequently, UE1 may not receive a messageindicating successful timing verification and thus fail the randomaccess procedure. Alternatively, the eNB may receive and decode thetiming verification message and determine that the timing advance usedby UE1 is incorrect. Consequently, UE1 may either not receive a messageindicating successful timing verification and thus fail the randomaccess procedure, or it may receive a message indicating timingverification failure (1004) and thus fail the random access procedure.UE1 then discontinues use of the timing advance indicated in thededicated RAR message.

The eNB may optionally transmit an RAR message on the downlink of theSCell (1012). The PDCCH of the RAR message 1012 is addressed to abroadcast RNTI (such as a random access RNTI). The RAR message 1012 caninclude timing advance information and an uplink grant. The timingadvance value is computed based on the received preamble X at the eNBand is the same value included in the dedicated RAR message 1002. TheRAR message 1012 may also indicate that it is a response to transmissionof preamble X. UE2 may receive the RAR message 1012 and apply the timingadvance. UE2 may then transmit a contention resolution message 1013. Thecontention resolution message can include a unique identifier of UE2.The eNB transmits to UE2 a message indicating successful contentionresolution (1014). Consequently, UE2 successfully completes its randomaccess procedure.

According to another embodiment, illustrated in FIG. 11, the UE 1103 maybe configured to operate on a PCell and an SCell. The PCell has adownlink frequency F1 DL (1111) and an uplink frequency F1 UL (1112).The SCell has a downlink frequency F2 DL (1121) and an uplink frequencyF2 UL (1122). The PCell downlink transmission and uplink reception maybe performed by an eNB (1101). The SCell downlink transmission anduplink reception may be performed by an RRH (1102) associated with theeNB (1101). The RRH (1102) may be connected to the eNB (1101) via ahigh-speed wired link (1104) that enables the eNB to perform schedulingof transmissions on the SCell. Thus, the RRH (1102) can function as aremote antenna of the eNB (1101).

The UE detects a transmission (1131) on F1 DL. The detected transmission(1131) includes a control channel component addressed to a uniqueidentifier of the UE. The UE decodes the control channel (1132) anddetermines that the control channel indicates resources for a datachannel (1133) in the transmission 1131. The UE decodes the data channel(1133) and obtains a message (1141). The message 1141 includes an uplinkresource grant 1143. The message 1141 may also include timing advanceinformation 1142. The UE transmits a message on the F2 UL using theuplink resource indicated in the uplink resource grant 1143.

According to an embodiment, illustrated in FIG. 12, a UE implementationmay include a physical layer 1211 and a medium access control (MAC)layer 1221. The physical layer can be configured to receive signals onat least a downlink frequency F1 (1231) and a downlink frequency F2(1232). The physical layer can also be configured to transmit signals onat least an uplink frequency F1 (1233) and an uplink frequency F2(1234). Further, the UE can be configured to apply different timingadvance values to transmissions on uplink frequency F1 and uplinkfrequency F2. The physical layer can also include logic 1213 to receivetransport blocks from the MAC layer 1221. The logic 1213 also determineswhether a transport block is to be transmitted on the F1 uplinkfrequency or the F2 uplink frequency.

The UE may receive a transmission 1201 on the F1 downlink frequency. Thetransmission 1201 may comprise a control channel and a data channel. Thephysical layer 1211 decodes the control channel and obtains the datachannel 1214. The physical layer transfers the received transport blockin the data channel to the MAC layer 1212. The MAC layer retrieves amessage 1222 from the transport block. The MAC layer decodes an uplinkresource grant field in the message 1222. The MAC layer constructs amessage 1224 for transmission by the UE. The MAC layer provides the atransport block including the message 1224 to the physical layer (1225).The MAC layer also provides to the physical layer the uplink resourcegrant information derived from the uplink resource grant field in themessage 1222. Further, the MAC layer indicates to the logic 1213 in thephysical layer that the message 1224 is to be transmitted on the uplinkfrequency F2 using the uplink resource grant information derived fromthe uplink resource grant field in the message 1222. The physical layerperforms a transmission of the transport block received from the MAClayer on the uplink frequency F2 using the resources according to theuplink resource grant information.

According to an embodiment, a mobile station is configured for operationon at least a first cell and a second cell, the first cell having afirst downlink frequency and a first uplink frequency, and the secondcell having a second downlink frequency and a second uplink frequency.The mobile station receives a control channel transmission addressed toa unique identifier of the mobile station. The mobile station determinesthat the control channel transmission indicates resources for a datachannel. The mobile station then receives a data channel transmission,the data channel transmission comprising a first message, the firstmessage indicating a first resource for an uplink transmission. Themobile station then transmits, on the second uplink frequency, a secondmessage using the first resource. The mobile station may receive thecontrol channel transmission addressed to the unique identifier of themobile station in response to a random access channel transmission. Thefirst message may further include timing advance information. The mobilestation may apply a timing advance derived from the timing advanceinformation, to the second uplink frequency. The mobile station may failto receive the second message. In response to determining that thesecond message is not received, the mobile station may disable thetiming advance applied to transmission on the second uplink frequency.

According to another embodiment a mobile station implements a mediumaccess control (MAC) protocol layer and physical layer. The mobilestation receives, at the physical layer, a first message addressed to aunique identifier of the mobile station, the first message comprising aMAC protocol layer packet. The mobile station decodes from the MACprotocol layer packet an uplink resource allocation. The MAC layerconstructs a second message for transmission by the physical layer. TheMAC layer submits the second message to the physical layer, indicatingthe frequency on which the second message is to be transmitted.

While the invention has been particularly shown and described withreference to particular embodiments, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention. Itis intended that such changes come within the scope of the followingclaims:

1. A method in a mobile station for performing random access,comprising: receiving, from a first cell, a message identifying a randomaccess preamble to transmit to a second cell, wherein the first andsecond cell operate on different frequencies; determining whether therandom access preamble is from a set of contention based random accesspreambles; receiving a random access response message, wherein therandom access response message is addressed to a unique identifier ofthe mobile station; and transmitting, if the identified random accesspreamble is from the set of contention based preambles, a timingverification message.
 2. The method according to claim 1 wherein thetiming verification message is transmitted to the second cell.
 3. Themethod according to claim 1 wherein determining whether the randomaccess preamble is from a set of contention based preambles comprisesreceiving system information of the second cell; and determining whetherthe random access preamble is from the set of contention based preamblesof the second cell.
 4. The method according to claim 1 whereintransmitting a timing verification message further comprises: receiving,in the random access response message, timing advance information;configuring a timing advance for the uplink frequency of the secondcell, wherein the timing advance is derived from the timing advanceinformation; and transmitting the timing verification message on theuplink frequency of the second cell.
 5. The method according to claim 4further comprising: receiving, subsequent to transmission of the timingverification message, a timing confirmation message; and applying thetiming advance to all transmissions on the uplink frequency of thesecond cell.
 6. The method according to claim 4 further comprising:failing to receive, subsequent to transmission of the timingverification message, a timing confirmation message; and disabling thetiming advance for all transmissions on the uplink frequency of thesecond cell.
 7. The method according to claim 6 wherein failing toreceive a timing confirmation message includes failing to receive atiming confirmation message for a specified duration.
 8. A method in abase station for performing random access by a mobile station,comprising: configuring the mobile station for operation on at least afirst cell and a second cell; transmitting, on the first cell, a messageassigning to the mobile station a random access preamble for performingrandom access on the second cell, wherein the random access preamble isa contention based random access preamble of the second cell; detectinga transmission of the random access preamble on the second cell; andtransmitting a message addressed to a unique identifier of the mobilestation, indicating a timing advance and an uplink resource grant,wherein the timing advance is determined based on the detectedtransmission of the random access preamble.
 9. The method according toclaim 8 wherein the uplink resource grant grants resources for uplinktransmission on the second cell.
 10. The method according to claim 8further comprising: transmitting, on the second cell, a messageaddressed to a common identifier, indicating a timing advance and anuplink resource grant, wherein the uplink resource grant grantsresources for uplink transmission on the second cell.
 11. The methodaccording to claim 8 further comprising: detecting a transmission by themobile station in the uplink resources indicated in the uplink resourcegrant; determining whether the timing advance used for the transmissionby the mobile station is correct; and transmitting, if the timingadvance used for the transmission by the mobile station is correct, amessage indicating a confirmation of timing.
 12. The method according toclaim 11 further comprising: transmitting, if the timing advance usedfor the transmission by the mobile station is incorrect, a messageindicating a timing failure.
 13. The method according to claim 11wherein determining whether the timing advance used for the transmissionby the mobile station is correct includes determining that thetransmission does not cause significant interference to other uplinktransmissions.
 14. The method according to claim 11 wherein determiningwhether the timing advance used for the transmission by the mobilestation is correct includes: determining a first time at which thetransmission by the mobile station in the uplink resources indicated inthe uplink resource grant is expected; and determining whether the timeat which the transmission by the mobile station in the uplink resourcesindicated in the uplink resource grant is received is substantially sameas the first time.